1. Field of the Invention
This invention relates generally to systems for focusing and deflecting electron beams, for example, as in vidicon or image orthicon tubes, and more particularly is directed to an improved electron-optical system of the mixed field type in which a magnetic field focuses a projected electron image of the system object onto a target while simultaneously an electric field deflects the image across the target area.
2. Description of the Prior Art
There have been ever increasing needs for high quality electron-optical systems in many fields. In the case of video camera tubes employing beams of low velocity electrons, electron-optical characteristics, such as, beam spot size, landing error and deflection linearity are of prime importance, and the size and power consumption of the tube are also of importance particularly when the tube is to be incorporated in a portable camera.
From the design standpoint, focus and deflection systems can be provided with any combinations of magnetic and electrostatic or electric fields. An all-magnetic system is used in nearly all conventional vidicons, and the development of this system seems to have progressed as far as possible. For much better performance of the electron-optical system, a new approach seems to be dictated. Generally, magnetic focusing by a long solenoid coil has a higher resolution capability than electrostatic focusing. This results from the relatively smaller aberration of the magnetic lens. On the other hand, electric or electrostatic deflection is advantageous over magnetic deflection in terms of the relatively reduced size and weight of an electrostatic yoke and its lower power consumption.
Therefore, it has been proposed, for example, as disclosed in detail in U.S. Pat. No. 3,319,110, issued May 9, 1967, to provide an electron-optical system of the mixed field type, that is, in which a substantially coaxial electrostatic yoke and solenoid extend along an envelope intermediate a beam source and a target. The solenoid generates a substantially constant magnetic field within and along the axis of the envelope, while the electrostatic yoke generates a variable electric field orthogonal to the magnetic field and capable of causing deflection of the electron beam along two coordinates lying in a plane orthogonal to the enevelope axis. The crossed electric and magnetic fields constitute a so-called "focus projection and scanning" or "FPS" cavity by which a projected electron image of an object of the system defined by an aperture or cross-over of the electron beam is focused on the target structure and, simultaneously, the image is deflected across the target area in accordance with signals applied to the electrostatic yoke. The foregoing electron-optical system is theoretically capable of providing high image resolution and high beam current density with minimum power requirements, size and weight, and with the electron beam, after deflection, traveling along a path parallel with its original path to enable normal or orthogonal landing of the beam on the target. The beam landing angle affects shading, resolution, lag and flicker of camera tubes employing low-velocity electron beams. Normal beam landing is especially necessary in a single tube color camera for obtaining the accurate read out of coded color signals therefrom.
The achievement of normal beam landing in a mixed field electron-optical system, as described above, requires that each of the magnetic and electric fields by precisely uniform. However, as a practical matter, it is not possible to provide magnetic and electric fields that are precisely uniform, particularly in the direction along the envelope axis, with the result that significant and undesirable landing errors occur.